College Park, Maryland      June 6 - 10 , 2004

MP49: Quasi-elastic Neutron Scattering Study of the Diffusion of Methyl Iodide Confined in the GelTech-200

Y.J. Glanville, P.E. Sokol (Pennsylvania State University, Dept. of Physics, University Park, PA, 16802), R.M. Dimeo (NIST Center for Neutron Research), C.M. Brown (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland)

The quasi-elastic scattering from methyl iodide confined in the pores of GelTech glass has been measured on the Fermi Chopper Spectrometer at NIST. Methyl Iodide is a quantum rigid rotor with a bulk freezing temperature of 207 K. Measurements on cooling were carried out at an incident energy of 2.5 meV with an energy resolution of 90 meV, with Q values ranging from 0.68 to 2.45 -1, and temperatures ranging from 215 K to 150 K. The measured QENS spectra could be split into three distinct regions based on their Q dependence. The high temperature scattering, above 205 K, shows a distinct Q2 dependence indicating a liquid like jump diffusion. At 203 K there is a very dramatic change in the scattering and we observe a change from a quadratic to a linear Q dependence. We believe this is a transition from a liquid to a geometrically confined solid phase, which has a structure like that of a glass. This result is substantiated by x-ray scattering measurements preformed by our group. At this sample temperature the x-ray spectra showed a Debye-Waller type dampening at high Q indicative of a solid. As the temperature was lowered further a transition occurred where the broadening was independent of Q. We wish to acknowledge the support of this research by the funding of the Department of Energy through grant DE-FG02-01ER45912, and the Department of Commerce through their support of the NIST Center for Neutron Scattering.

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